System and method for migrating storage while in use

ABSTRACT

A system and method include receiving a request to transfer data from a first storage space to a second storage space. The system and method also include creating an access table and a location table. The access table includes one or more access values indicative of data being accessed. The location table includes one or more location values indicative of a location of the data in the first storage space or the second storage space. The system and method also include transferring data between the first storage space and the second storage space using the access table and the location table. The data is accessible on both the first storage device the second storage device based on the one or more location values of the location table and access to the data is based on the one or more access values of the access table.

BACKGROUND

The following description is provided to assist the understanding of thereader. None of the information provided or references cited is admittedto be prior art.

Virtual computing systems are widely used in a variety of applications.Virtual computing systems include one or more host machines running oneor more virtual machines concurrently. The one or more virtual machinesutilize the hardware resources of the underlying one or more hostmachines. Each virtual machine may be configured to run an instance ofan operating system. Modern virtual computing systems allow severaloperating systems and several software applications to be safely run atthe same time on the virtual machines of a single host machine, therebyincreasing resource utilization and performance efficiency. Each virtualmachine is managed by a hypervisor or virtual machine monitor.Occasionally, data for a virtual machine may be migrated from a firststorage space to a second storage space, such as for maintenance of thefirst machine, utilization of particular resources of a second hostmachine, migration of a virtual machine, etc. Typically, a copy of thedata in the first storage space is made, either directly to thedestination storage space or to an interim storage space, such as ashared storage. However, in each of these instances, a full copy of thestorage space is made and/or use of the original storage space must beceased otherwise the resulting copy of the data will differ from thein-use data. Presented herein is a method for transferring data of astorage space while permitting use of the data.

SUMMARY

In accordance with at least some aspects of the present disclosure, amethod is disclosed. The method includes receiving, by a storage spacetransfer system, a request to transfer data from a first storage spaceto a second storage space. The method includes creating, by the storagespace transfer system, an access table and a location table, the accesstable comprising one or more access values indicative of data beingaccessed, the location table comprising one or more location valuesindicative of a location of the data in the first storage space or thesecond storage space. The method also includes transferring data betweenthe first storage space and the second storage space using the accesstable and the location table. A first portion of data is accessible onthe first storage device and a second portion of data is accessible onthe second storage device based on the one or more location values ofthe location table. Access to the data is based on the one or moreaccess values of the access table.

In accordance with another aspect of the present disclosure, anothermethod is disclosed. The method includes receiving, by a storage spacetransfer system, a request to transfer data from a first storage spaceto a second storage space. The method includes creating, by the storagespace transfer system, an access table and a location table, the accesstable comprising one or more access values indicative of data beingaccessed, the location table comprising one or more location valuesindicative of a location of the data in the first storage space or thesecond storage space. The method also includes accessing an access rangeof the access table and determining the access range is not in use basedon one or more access values in the access range. The method furtherincludes determining a location range corresponding to the access rangeis not in the second storage space based on one or more location valuesin the location range and setting the one or more access values in theaccess range to an access value indicative of the underlying data beingin use. The method includes transferring data for the access rangebetween the first storage space and the second storage space andupdating the one or more location values in the location range to beindicative of the data stored in the second storage space. The methodstill further includes resetting the one or more access values in theaccess range to an access value indicative of the underlying data notbeing in use. A first portion of data is accessible on the first storagedevice and a second portion of data is accessible on the second storagedevice based on the one or more location values of the location table.

In accordance with some other aspects of the present disclosure, asystem is disclosed. The system includes a storage space transfer systemin a virtual computing system. The storage space transfer system isconfigured to receive a request to transfer data from a first storagespace to a second storage space; create an access table and a locationtable, the access table comprising one or more access values indicativeof data being accessed, the location table comprising one or morelocation values indicative of a location of the data in the firststorage space or the second storage space; and transfer data between thefirst storage space and the second storage space using the access tableand the location table. A first portion of data is accessible on thefirst storage device and a second portion of data is accessible on thesecond storage device based on the one or more location values of thelocation table. Access to the data is based on the one or more accessvalues of the access table.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the following drawings and thedetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a virtual computing system, in accordancewith some embodiments of the present disclosure.

FIG. 2 is a block diagram of a storage space transfer system of thevirtual computing system of FIG. 1, in accordance with some embodimentsof the present disclosure.

FIG. 3 is block diagram of an example access table, in accordance withsome embodiments of the present disclosure.

FIG. 4 is block diagram of an example location table, in accordance withsome embodiments of the present disclosure.

FIG. 5 is an example flowchart outlining operations for transferringdata of a storage space while permitting use of the virtual computingsystem of FIG. 1, in accordance with some embodiments of the presentdisclosure.

FIG. 6 is an example flowchart outlining operations for accessing dataof two storage spaces while permitting use of the virtual computingsystem of FIG. 1, in accordance with some embodiments of the presentdisclosure.

The foregoing and other features of the present disclosure will becomeapparent from the following description and appended claims, taken inconjunction with the accompanying drawings. Understanding that thesedrawings depict only several embodiments in accordance with thedisclosure and are, therefore, not to be considered limiting of itsscope, the disclosure will be described with additional specificity anddetail through use of the accompanying drawings.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, and designed in awide variety of different configurations, all of which are explicitlycontemplated and make part of this disclosure.

The present disclosure is generally directed to a virtual computingsystem having a plurality of clusters, with each cluster having aplurality of nodes. Each of the plurality of nodes includes one or morevirtual machines managed by an instance of a hypervisor. Each virtualmachine uses a storage space to store and operate on data. Occasionally,data stored in a first storage space may be moved to a second storagespace. For instance, a first storage space may be used for normal usagewhile a second storage space may be optimized for high performance, suchas modern low-latency technologies such as NVMe or PMEM. In otherinstances, maintenance may be performed on the first storage space, suchas replacement of a physical disk drive, etc. In still other instances,moving data from the first storage space to the second storage space maybe done when migrating a virtual machine from a first host machine to asecond host machine. The virtual machine may be managed by a hypervisor,such as an AHV type of hypervisor provided by Nutanix, Inc.

Conventionally, data stored in a storage space for the virtual machineis copied directly to a destination storage space or, in some instances,to a shared storage space. In such situations, use and/or modificationof the data is ceased while the copy occurs. Such a disruption reducescan affect users of virtual machines when the data to be transferred islarge and/or network conditions result in a slow data transfer rate.This downtime of virtual machines requires scheduling data storagetransfers at specific “maintenance” times, which may not particularly beconvenient.

Regardless of the inconvenience of scheduling the data storage transfer,the operation of a virtual machine can also suffer as the virtualmachine may be inoperable during the data storage transfer. Theperformance of the virtual machine can suffer and prevent the virtualmachine from performing certain tasks during the data storage transfer.Thus, a technical problem currently exists in which users, such asvirtual machine users, are prevented from operating efficiently duringdata storage transfers.

As a result, some systems have implemented alternative data transferalgorithms to permit usage of data while the data is being used ormodified. Such data transfer algorithms include snapshotting data, dirtydata tracking, and/or convergence algorithms to track what data is beingmodified and then iteratively transferring successive modified data.That is, a common method involves copying the data from first storagespace to a destination or second storage space while keeping track ofany modifications made to the source data as a result of the data beingin use (i.e., dirty tracking). Once the initial copy completes, anotheriteration starts and copies only the data that has since been modified.Such methods rely on either the copying of the data to be fast enough toapproach a converged state or require throttling on the virtual machineaccessing the data to achieve a similar effect. When such a convergedstate is reached, access to the data is suspended for a final copy fromthe first storage space to the second, destination storage space. Such adata transfer algorithm may be difficult to implement if a large amountof data is modified during each iteration, thereby resulting in slow orno convergence. When a slow or no convergence situation occurs, theunderlying performance is stunted to allow the data transfer toeventually converge. However, such stunting of performance can result inunacceptably slow system performance.

Another method involves making the source data “copy-on-write” toachieve a snapshot effect. That is, modifications made to the sourcedata are synchronously replicated to the destination data. In thebackground, an entire snapshot of the original source data is copied tothe destination. Upon completion, the source data and the snapshot ofthe original source data on the destination are commonly coalesced viathe synchronous replication in order to produce a flat view of the data.Variations of this method exist which also require convergence. Thisalternative data transfer algorithm may also be difficult to implementif a large amount of data is modified during each iteration, therebyresulting in continual synchronous replication of the data, therebydelaying the transfer for potentially unacceptably long periods of time.

The present disclosure provides an improved solution. For example, thepresent disclosure provides a migration tool that facilitatestransferring data from a first storage space to a second storage spacewhile significantly reducing the downtime to the user, such as a user ofa virtual machine. Specifically, the data transfer tool facilitates datastorage transfer such that only the data currently in use or currentlybeing copied is impacted and only for a short period of time. Data thatis not currently being copied or has already been transferred can beused and/or modified via normal operations.

Described herein is a method for transferring data from a first storagespace to a second, destination storage space without dirty tracking,convergence, or snapshots. In the proposed method herein, two tables,such as bitmaps or other tabular representations, are utilized. A firsttable, called a location table, indicates whether the latest version ofthe data is at the first storage space or at the second, destinationstorage space. A second table, called an access table, indicates whethera particular range of the dataset (e.g., a block device sector) iscurrently in use or being copied. The access table guards access to thedata and the corresponding regions of the location table when in use orbeing copied.

When data is being accessed, the corresponding range is automaticallyannotated on the access table. If any part of the range is alreadyannotated, then the data range is being copied by a copier. The methodallows the user, such as a virtual machine, to join a waiting list andsleep until further notice. If the full range is successfully annotated,the access can continue. For reads, the location table must be consultedto determine where the data must be read from. For writes, all data issent to the destination and the location table must be updatedaccordingly. Once the access completes, the range is cleared from theaccess table.

When data is being copied by the copier, the corresponding range isautomatically annotated on the access table. If any part of the range isalready annotated, then the data range is being accessed. The copierwill skip that range and attempt the next. If the full range issuccessfully annotated, then the copier checks the location table andcopies any data that is still present at the source. When the copycompletes, the location table is updated accordingly and the rangecleared from the access table. At this point, if a user process ispresent on a waiting list, a notification is sent to the user to bewoken up. The procedure continues until the entire location tableindicates that all data is present at the destination storage space.

The present disclosure provides an easy, time saving, and automaticprocess for transferring data from a first storage space to a secondstorage space while permitting a user to access and modify the datawithout substantial disruptions.

Referring now to FIG. 1, a virtual computing system 100 is shown, inaccordance with some embodiments of the present disclosure, though itshould be understood that the present disclosure is not limited to avirtual computing system 100 environment. The virtual computing system100 includes a plurality of nodes, such as a first node 105, a secondnode 110, and a third node 115. The first node 105 includes user virtualmachines (“user VMs”) 120A and 120B (collectively referred to herein as“user VMs 120”), a hypervisor 125 configured to create and run the userVMs, and a controller/service VM 130 configured to manage, route, andotherwise handle workflow requests between the various nodes of thevirtual computing system 100. Similarly, the second node 110 includesuser VMs 135A and 135B (collectively referred to herein as “user VMs135”), a hypervisor 140, and a controller/service VM 145, and the thirdnode 115 includes user VMs 150A and 150B (collectively referred toherein as “user VMs 150”), a hypervisor 155, and a controller/service VM160. The controller/service VM 130, the controller/service VM 145, andthe controller/service VM 160 are all connected to a network 165 tofacilitate communication between the first node 105, the second node110, and the third node 115. Although not shown, in some embodiments,the hypervisor 125, the hypervisor 140, and the hypervisor 155 may alsobe connected to the network 165.

The virtual computing system 100 also includes a storage pool 170. Thestorage pool 170 may include network-attached storage 175 anddirect-attached storage 180A, 180B, and 180C. The network-attachedstorage 175 may be accessible via the network 165 and, in someembodiments, may include cloud storage 185, as well as local storagearea network 190. In contrast to the network-attached storage 175, whichis accessible via the network 165, the direct-attached storage 180A,180B, and 180C may include storage components that are provided withineach of the first node 105, the second node 110, and the third node 115,respectively, such that each of the first, second, and third nodes mayaccess its respective direct-attached storage without having to accessthe network 165.

It is to be understood that only certain components of the virtualcomputing system 100 are shown in FIG. 1. Nevertheless, several othercomponents that are needed or desired in the virtual computing system toperform the functions described herein are contemplated and consideredwithin the scope of the present disclosure. Additional features of thevirtual computing system 100 are described in U.S. Pat. No. 8,601,473,the entirety of which is incorporated by reference herein.

Although three of the plurality of nodes (e.g., the first node 105, thesecond node 110, and the third node 115) are shown in the virtualcomputing system 100, in other embodiments, greater than or fewer thanthree nodes may be used. Likewise, although only two of the user VMs(e.g., the user VMs 120, the user VMs 135, and the user VMs 150) areshown on each of the respective first node 105, the second node 110, andthe third node 115, in other embodiments, the number of the user VMs oneach of the first, second, and third nodes may vary to include either asingle user VM or more than two user VMs. Further, the first node 105,the second node 110, and the third node 115 need not always have thesame number of the user VMs (e.g., the user VMs 120, the user VMs 135,and the user VMs 150). Additionally, more than a single instance of thehypervisor (e.g., the hypervisor 125, the hypervisor 140, and thehypervisor 155) and/or the controller/service VM (e.g., thecontroller/service VM 130, the controller/service VM 145, and thecontroller/service VM 160) may be provided on the first node 105, thesecond node 110, and/or the third node 115.

In some embodiments, each of the first node 105, the second node 110,and the third node 115 may be a hardware device, such as a server. Forexample, in some embodiments, one or more of the first node 105, thesecond node 110, and the third node 115 may be an NX-1000 server,NX-3000 server, NX-6000 server, NX-8000 server, etc. provided byNutanix, Inc. or server computers from Dell, Inc., Lenovo Group Ltd. orLenovo PC International, Cisco Systems, Inc., etc. In other embodiments,one or more of the first node 105, the second node 110, or the thirdnode 115 may be another type of hardware device, such as a personalcomputer, an input/output or peripheral unit such as a printer, or anytype of device that is suitable for use as a node within the virtualcomputing system 100. In some embodiments, the virtual computing system100 may be part of a data center.

Each of the first node 105, the second node 110, and the third node 115may also be configured to communicate and share resources with eachother via the network 165. For example, in some embodiments, the firstnode 105, the second node 110, and the third node 115 may communicateand share resources with each other via the controller/service VM 130,the controller/service VM 145, and the controller/service VM 160, and/orthe hypervisor 125, the hypervisor 140, and the hypervisor 155. One ormore of the first node 105, the second node 110, and the third node 115may also be organized in a variety of network topologies, and may betermed as a “host” or “host machine.”

Also, although not shown, one or more of the first node 105, the secondnode 110, and the third node 115 may include one or more processingunits configured to execute instructions. The instructions may becarried out by a special purpose computer, logic circuits, or hardwarecircuits of the first node 105, the second node 110, and the third node115. The processing units may be implemented in hardware, firmware,software, or any combination thereof. The term “execution” is, forexample, the process of running an application or the carrying out ofthe operation called for by an instruction. The instructions may bewritten using one or more programming language, scripting language,assembly language, etc. The processing units, thus, execute aninstruction, meaning that they perform the operations called for by thatinstruction.

The processing units may be operably coupled to the storage pool 170, aswell as with other elements of the first node 105, the second node 110,and the third node 115 to receive, send, and process information, and tocontrol the operations of the underlying first, second, or third node.The processing units may retrieve a set of instructions from the storagepool 170, such as, from a permanent memory device like a read onlymemory (ROM) device and copy the instructions in an executable form to atemporary memory device that is generally some form of random accessmemory (RAM). The ROM and RAM may both be part of the storage pool 170,or in some embodiments, may be separately provisioned from the storagepool. Further, the processing units may include a single stand-aloneprocessing unit, or a plurality of processing units that use the same ordifferent processing technology.

With respect to the storage pool 170 and particularly with respect tothe direct-attached storage 180A, 180B, and 180C, each of thedirect-attached storage may include a variety of types of storagedevices. For example, in some embodiments, one or more of thedirect-attached storage 180A, 180B, and 180C may include, but is notlimited to, any type of RAM, ROM, flash memory, magnetic storage devices(e.g., hard disk, floppy disk, magnetic strips, etc.), optical disks(e.g., compact disk (CD), digital versatile disk (DVD), etc.), smartcards, solid state devices, etc. Likewise, the network-attached storage175 may include any of a variety of network accessible storage (e.g.,the cloud storage 185, the local storage area network 190, etc.) that issuitable for use within the virtual computing system 100 and accessiblevia the network 165. The storage pool 170 including the network-attachedstorage 175 and the direct-attached storage 180A, 180B, and 180C maytogether form a distributed storage system configured to be accessed byeach of the first node 105, the second node 110, and the third node 115via the network 165, the controller/service VM 130, thecontroller/service VM 145, and the controller/service VM 160, and/or thehypervisor 125, the hypervisor 140, and the hypervisor 155. In someembodiments, the various storage components in the storage pool 170 maybe configured as virtual disks for access by the user VMs 120, the userVMs 135, and the user VMs 150.

Each of the user VMs 120, the user VMs 135, and the user VMs 150 is asoftware-based implementation of a computing machine in the virtualcomputing system 100. The user VMs 120, the user VMs 135, and the userVMs 150 emulate the functionality of a physical computer. Specifically,the hardware resources, such as processing unit, memory, storage, etc.,of the underlying computer (e.g., the first node 105, the second node110, and the third node 115) are virtualized or transformed by therespective hypervisor 125, the hypervisor 140, and the hypervisor 155,respectively, into the underlying support for each of the user VMs 120,the user VMs 135, and the user VMs 150 that may run its own operatingsystem and applications on the underlying physical resources just like areal computer. By encapsulating an entire machine, including CPU,memory, operating system, storage devices, and network devices, the userVMs 120, the user VMs 135, and the user VMs 150 are compatible with moststandard operating systems (e.g. Windows, Linux, etc.), applications,and device drivers. Thus, each of the hypervisor 125, the hypervisor140, and the hypervisor 155 is a virtual machine monitor that allows asingle physical server computer (e.g., the first node 105, the secondnode 110, third node 115) to run multiple instances of the user VMs 120,the user VMs 135, and the user VMs 150, with each user VM sharing theresources of that one physical server computer, potentially acrossmultiple environments. By running the user VMs 120, the user VMs 135,and the user VMs 150 on each of the first node 105, the second node 110,and the third node 115, respectively, multiple workloads and multipleoperating systems may be run on a single piece of underlying hardwarecomputer (e.g., the first node, the second node, and the third node) toincrease resource utilization and manage workflow.

The user VMs 120, the user VMs 135, and the user VMs 150 are controlledand managed by their respective instance of the controller/service VM130, the controller/service VM 145, and the controller/service VM 160.The controller/service VM 130, the controller/service VM 145, and thecontroller/service VM 160 are configured to communicate with each othervia the network 165 to form a distributed system 195. Each of thecontroller/service VM 130, the controller/service VM 145, and thecontroller/service VM 160 may also include a local management system(e.g., Prism Element from Nutanix, Inc.) configured to manage varioustasks and operations within the virtual computing system 100. Forexample, as discussed below, in some embodiments, the local managementsystem of the controller/service VM 130, the controller/service VM 145,and the controller/service VM 160 may facilitate transfer of data from afirst storage space to a second storage space. In other implementations,each VM 120A, 120B, 135A, 135B, 150A, 150B may facilitate the transferof data from a first storage space to a second storage space. In otherimplementations, each hypervisor 125, 140, 155 may facilitate thetransfer of data from a first storage space to a second storage space.In still other implementations, an external system may facilitate thetransfer of data from a first storage space to a second storage space.

The hypervisor 125, the hypervisor 140, and the hypervisor 155 of thefirst node 105, the second node 110, and the third node 115,respectively, may be configured to run virtualization software, such as,ESXi from VMWare, AHV from Nutanix, Inc., XenServer from Citrix Systems,Inc., etc., for running the user VMs 120, the user VMs 135, and the userVMs 150, respectively, and for managing the interactions between theuser VMs and the underlying hardware of the first node 105, the secondnode 110, and the third node 115. Each of the controller/service VM 130,the controller/service VM 145, the controller/service VM 160, thehypervisor 125, the hypervisor 140, and the hypervisor 155 may beconfigured as suitable for use within the virtual computing system 100.

The network 165 may include any of a variety of wired or wirelessnetwork channels that may be suitable for use within the virtualcomputing system 100. For example, in some embodiments, the network 165may include wired connections, such as an Ethernet connection, one ormore twisted pair wires, coaxial cables, fiber optic cables, etc. Inother embodiments, the network 165 may include wireless connections,such as microwaves, infrared waves, radio waves, spread spectrumtechnologies, satellites, etc. The network 165 may also be configured tocommunicate with another device using cellular networks, local areanetworks, wide area networks, the Internet, etc. In some embodiments,the network 165 may include a combination of wired and wirelesscommunications.

Referring still to FIG. 1, in some embodiments, one of the first node105, the second node 110, or the third node 115 may be configured as aleader node. The leader node may be configured to monitor and handlerequests from other nodes in the virtual computing system 100. Theleader node may also be configured to receive and handle requests (e.g.,user requests) from outside of the virtual computing system 100. If theleader node fails, another leader node may be designated. Furthermore,one or more of the first node 105, the second node 110, and the thirdnode 115 may be combined together to form a network cluster (alsoreferred to herein as simply “cluster.”) Generally speaking, all of thenodes (e.g., the first node 105, the second node 110, and the third node115) in the virtual computing system 100 may be divided into one or moreclusters. One or more components of the storage pool 170 may be part ofthe cluster as well. For example, the virtual computing system 100 asshown in FIG. 1 may form one cluster in some embodiments. Multipleclusters may exist within a given virtual computing system (e.g., thevirtual computing system 100). The user VMs 120, the user VMs 135, andthe user VMs 150 that are part of a cluster can be configured to shareresources with each other. In some embodiments, multiple clusters mayshare resources with one another.

Further, in some embodiments, although not shown, the virtual computingsystem 100 includes a central management system (e.g., Prism Centralfrom Nutanix, Inc.) that is configured to manage and control theoperation of the various clusters in the virtual computing system. Insome embodiments, the central management system may be configured tocommunicate with the local management systems on each of thecontroller/service VM 130, the controller/service VM 145, thecontroller/service VM 160 for controlling the various clusters.

Again, it is to be understood again that only certain components of thevirtual computing system 100 are shown and described herein.Nevertheless, other components that may be needed or desired to performthe functions described herein are contemplated and considered withinthe scope of the present disclosure. It is also to be understood thatthe configuration of the various components of the virtual computingsystem 100 described above is only an example and is not intended to belimiting in any way. Rather, the configuration of those components mayvary to perform the functions described herein.

Turning to FIG. 2, a block diagram of a data transfer system 200 isshown, in accordance with some embodiments of the present disclosure.The data transfer system 200 is used to transfer data stored in a firststorage space, such as storage space A 218 to a second storage space,such as storage space B 228. While the implementation shown has storagespace B 228 on a separate host machine B 220, it should be understoodthat the second storage space, storage space B 228, can be a separatestorage space on the same storage device 216 as the first storage space,can be on a separate storage device on the same host machine A 210,and/or may be on a shared storage space, such as the storage pool 170.

The data transfer system 200 facilitates the transfer of data from afirst storage space to a second storage space while permitting the user,such as a virtual machine 214 or 224, to access the underlying dataduring the data transfer so that any downtime of the virtual machine oran application accessing the data during the data transfer issignificantly reduced. For example, as discussed above, conventionalmechanisms entail a downtime and/or reduced performance to permitmodified data to be transferred through an iterative process and/or slowtransfer because of duplicative synchronous data modifications. The datatransfer system 200 facilitates the data transfer using an access table262 and a location table 264 to allow a user, such as a virtual machine,to access data stored in two different storage spaces 218, 228 while thedata transfer is on-going. By reducing the downtime, the presentdisclosure optimizes the functioning of host machines and/or the virtualmachines running thereon and increases user satisfaction.

Host machine A 210 and host machine B 220 can be analogous to the firstnode 105 and the second node 110, respectively, discussed with respectto FIG. 1 above. Although each of host machine A 210 and host machine B220 have been shown as having only their respective hypervisors (e.g.,hypervisor 212 and hypervisor 222, respectively) and their respectivevirtual machines (e.g., virtual machines 214 and virtual machines 224,respectively), each of host machine A 210 and host machine B 220 canhave additional components (e.g., the controller/service VM), asdiscussed above. Further, the number of the virtual machines 214, 224,on each of host machine A 210 and host machine B 220 may vary from oneanother, as also discussed above.

As shown in FIG. 2, host machine A 210 includes a storage device 216with an allocated storage space A 218. The allocated storage space A 218can be an allocated data storage space for a virtual machine of thevirtual machines 214 operating on host machine A 210. The storage device216 may include, but is not limited to, any type of magnetic storagedevices (e.g., hard disk, floppy disk, magnetic strips, etc.), opticaldisks (e.g., compact disk (CD), digital versatile disk (DVD), etc.),smart cards, solid state devices, etc. Similarly, host machine B 220includes a storage device 226. The storage device 226 may include, butis not limited to, any type of magnetic storage devices (e.g., harddisk, floppy disk, magnetic strips, etc.), optical disks (e.g., compactdisk (CD), digital versatile disk (DVD), etc.), smart cards, solid statedevices, etc. In some instances, the data stored in storage space A 218may be transferred or migrated to host machine B 220. Such a transferalmay be to facilitate maintenance of the storage device 216, to migrateone or more of the virtual machines 214 to host machine B 220, and/or toutilize a different storage device 226, such as a high performancelow-latency storage device.

When a data transfer is to be performed, a new storage space B 228 canbe allocated as the second, destination storage space. In someimplementations, the new storage space B 228 can be on the storagedevice 216 of the host machine A 210, on the storage device 226 of asecond host machine, such as host machine B 220, or on a shared storage,such as storage pool 170 of FIG. 1. While only two host machines 210,220 are shown in FIG. 2, in other embodiments, the number of hostmachines may vary from fewer than two to greater than two.

To facilitate the transfer of data from a first storage space to asecond storage space while permitting, the data transfer system 200includes a storage space transfer tool 250. In some implementations, thestorage space transfer tool 250 is implemented on another device, suchas a central management system. In other implementations, the storagespace transfer tool 250 can be implemented as part of a hypervisor, suchas hypervisor 212 or 222. In still other implementations, the storagespace transfer tool 250 may be a part of a virtual machine 214, 224. Instill other embodiments, the storage space transfer tool 250 may be partof a controller/service VM (e.g., the controller/service VM 130, thecontroller/service VM 145, the controller/service VM 160 of FIG. 1).Specifically, when part of the controller/service VM (e.g., thecontroller/service VM 130, the controller/service VM 145, thecontroller/service VM 160), the storage space transfer tool 250 mayreside within the local management system (e.g., Prism Element) of thecontroller/service VM. Further, in some embodiments, an instance of thestorage space transfer tool 250 may be located on the controller/serviceVM of each node (e.g., the first node 105, the second node 110, and thethird node 115). In yet other embodiments, the storage space transfertool 250 may be part of another component within or associated with thevirtual computing system (e.g., the virtual computing system 100). Thus,the location of the storage space transfer tool 250 within the virtualcomputing system may vary from one embodiment to another.

Further, although not shown, the storage space transfer tool 250 may beconfigured as hardware, software, firmware, or a combination thereof.Specifically, the storage space transfer tool 250 may include one ormore processing units configured to execute instructions and one or morememory units to store those instructions and other conversion relateddata. In some embodiments, the storage space transfer tool 250 may beconnected to a storage pool (e.g., the storage pool 170) to receive,send, and process information, and to control the operations of the datatransfer. The instructions may be carried out by a special purposecomputer, logic circuits, or hardware circuits of the storage spacetransfer tool 250. The processing units may, thus, be implemented inhardware, firmware, software, or any combination thereof. The processingunits execute an instruction, meaning that they perform the operationscalled for by that instruction. The processing units may retrieve a setof instructions from a memory (e.g., the storage pool 170 or any othermemory associated with the migration tool in which such instructions maybe stored). For example, in some embodiments, the processing units mayretrieve the instructions from a permanent memory device like a readonly memory (ROM) device and copy the instructions in an executable formto a temporary memory device that is generally some form of randomaccess memory (RAM). The ROM and RAM may both be part of the storagepool (e.g., the storage pool 170), or in some embodiments, may beseparately provisioned from the storage pool. Further, the processingunits may include a single stand-alone processing unit, or a pluralityof processing units that use the same or different processingtechnology. The instructions may be written using one or moreprogramming language, scripting language, assembly language, etc.

Referring still to FIG. 2, the storage space transfer tool 250 includesa plurality of components for facilitating the data transfer from thefirst storage space to the second storage space. For example, thestorage space transfer tool 250 includes a storage space transfer system260 and, optionally, a user interface 270. Similar to the storage spacetransfer tool 250, the storage space transfer system 260 may beconfigured as hardware, software, firmware, or a combination thereofhaving one or more processing units configured to execute instructionsfor facilitating the data transfer from the first storage space to thesecond storage space.

The storage space transfer tool 250 includes a user interface 270. Theuser interface 270 is used to receive an input or data transferindication from a user to transfer or migrate data from the firststorage space to a second storage space. The user interface 270 maypresent one or more displays to the user presenting an option (e.g., asa menu item) to designate the first storage space to be transferred andthe second, destination storage space to which the data is to betransferred. The user may interact with the option to start the datatransfer process. Upon receiving the indication (e.g., input) from theuser to start the data transfer process, the user interface 270 may sendan indication to the storage space transfer system 260. In response toreceiving the indication from the user interface 260, the storage spacetransfer system 260 starts the process for transferring the data fromthe first storage space to the second storage space. In someimplementations, the storage space transfer system 260 can receive otherindications from other systems to automatically start the data transferprocess without utilizing user input via the user interface 270. Infurther embodiments, the user interface 270 can include diagnosticinterfaces, such as a visualization of the access table 262, avisualization of the location table 264, a status or progress indicatorfor the transfer process, data indicative of one or more valuesassociated with the transfer process (e.g., remaining amount of data tobe transferred, the amount of data transferred, a data transfer rate,etc.).

The storage space transfer system 260 implements the data transferprocess. In some implementations, the storage space transfer system 260can store the access table 262 and location table 264 during thetransfer process. In some implementations, the storage space transfersystem 260 includes a copier 266. In other implementations, the copier266 may be separate from the storage space transfer system 260. Thestorage space transfer system 260 identifies the first storage space 218storing the current data and a second, destination storage space 228. Ifthe second, destination storage space 228 is not established, thestorage space transfer system 260 can allocate a storage space for thesecond, destination storage space 228 on a particular storage device,such as the storage device 226 of a destination host machine B 210. Thestorage space transfer system 260 also creates two tables, such asbitmaps or other tabular representations, such as those shown in FIGS.3-4, for the access table 262 and the location table 264. The locationtable 264 indicates whether the latest version of the data is at thefirst storage space 218 or at the second, destination storage space 228.The access table 262 indicates whether a particular range of theto-be-transferred dataset (e.g., a block device sector) is currently inuse or being copied. If a particular dataset is indicated as in-use bythe access table 262, then access to the data and the correspondingregions of the location table 264 is prevented while in use or beingcopied.

When data is being accessed, the corresponding range is automaticallyannotated on the access table 262. As noted above, the access table 262can be any tabular format, including bitmaps. The representation of anexample access table 262 is shown in FIG. 3 having several access values300 representative of sections of data. The sections of data can beblock device sectors, bytes, files, or any other segment of data. Insome implementations, the sections of data represented by the accessvalues 300 can be different sizes. The access value 300 in the accesstable 262 for a particular section of data is assigned a bit value of 1if the corresponding section of the data is in use, either by a user orby the copier 266. Similarly, the access value 300 in the access table262 for a particular section of data is assigned a bit value of 0 if thecorresponding section of the data is not in use, either by a user or bythe copier 266.

If any part of an access range 310 is already annotated, then the datarange 310 is either currently in use or being copied by the copier 266.If the access range 310 is in use by a user, then the copier 266 canskip over the access range 310 until the access values 300 indicate theaccess range 310 is no longer in use. In some implementations, thecopier 266 can proceed through any remaining access ranges 310 andreturn to the skipped access range 310 after cycling through the lateraccess ranges 310 in the access table 262. If the access range 310 is inuse by the copier 266 and a request to access the data in the accessrange 310 is sent by a user, then the user, such as a virtual machine,can be set into a waiting list by the storage space transfer system 260and the request sleeps or waits until the copier 266 completes thecopying of the data for the access range 310. If a full access range 310is successfully annotated while the copier 266 is proceeding (e.g., theaccess range 310 is set to all 1 bit values), then access to the accessrange 310 and any access values 300 for particular sections of data canbe reset to 0 bit values to allow access to the underlying data sincethe copying to the second, destination storage space 228 has beencompleted.

If the requested data is not in use based on the access table 262, thenthe location table 264 is consulted. For any requests to write or modifythe underlying data, all data is sent to the second, destination storagespace 228 and the location table 264 is updated accordingly as discussedbelow. For any requests to read the underlying data, the location table264 is consulted to determine where the data must be read from, eitherfrom the first storage space 218 or the second, destination storagespace 228. Thus, the storage space transfer system described hereinadvantageously accesses both storage spaces 218, 228 during the datatransfer process to minimize data transfers, write/read operations,storage space utilization, and transfer time.

The location table 264 can be any tabular format, including bitmaps. Therepresentation of an example location table 264 is shown in FIG. 4having several location values 400 representative of sections of data.The sections of data can be block device sectors, bytes, files, or anyother segment of data. In some implementations, the sections of datarepresented by the location values 400 can be different sizes. Thelocation value 400 in the location table 264 for a particular section ofdata is assigned a bit value of 0 if the corresponding section of thedata is located at the first storage space 218. Similarly, the locationvalue 400 in the location table 264 for a particular section of data isassigned a bit value of 1 if the corresponding section of the data islocated at the second, destination storage space 228.

If any part of a location range 410 is already annotated, then the datarange 410 is either currently being copied by the copier 266 orcompletely moved to the second, destination storage space 228. If a fulllocation range 410 is successfully annotated while the copier 266 isproceeding (e.g., the location range 410 is set to all 1 bit values),then the underlying data have been completely copied to the second,destination storage space 228.

The data copier 266 access and updates both the access table 262 and thelocation table 264. When data is being copied by the copier 266, thecorresponding access values 300 of the relevant access range 310 areautomatically annotated by the copier 266 in the access table 262. Ifany part of the access range 310 is already annotated, then the accessrange 310 is being accessed. The copier 266 skips any access ranges 310that are in use and proceeds to the next access range 310. If the fullaccess range 310 is not in use, then the copier 266 checks the locationtable 264 to determine if the underlying data has been copied to thesecond, destination storage space 228. If the underlying data has notbeen copied, then the copier 266 sets the values 300 to in use (e.g., 1bit values) and copies any data that is still present at the firststorage space 218. When the copy completes, the location table 264 isupdated accordingly and the access range 310 and any access values 300for particular sections of data can be reset to 0 bit values to allowaccess to the underlying data since the copying to the second,destination storage space 228 has been completed. At this point, if auser process is present on a waiting list, a notification is sent to theuser to be woken up to access the underlying data from the second,destination storage space 264. The procedure continues until the entirelocation table 264 indicates that all data is present at the second,destination storage space 228.

It is to be understood that only some components of the storage spacetransfer tool 250 are shown and described herein. Nevertheless, othercomponents that are considered desirable or needed to perform thefunctions described herein are contemplated and considered within thescope of the present disclosure.

Turning now to FIG. 5, a flowchart outlining a process 500 fortransferring data of a storage space while permitting use, in accordancewith some embodiments of the present disclosure. The process 500 mayinclude additional, fewer, or different operations, depending on theparticular embodiment. Further, the process 500 is described inconjunction with FIGS. 2-4. Thus, the process 500 is used fortransferring data of a storage space while permitting use of anunderlying computing system, such as transferring data for a virtualcomputing system from a first storage space 218 to a second, destinationstorage space, such as second storage space 228.

The process 500 starts at operation 505 with the storage space transfersystem 260 of the storage space transfer tool 250 receiving a request totransfer a storage space from a first storage space to a second,destination storage space. In some implementations, the request can be auser request via the user interface 270 that identifies the firststorage space and the second, destination storage space. In otherimplementations, the request may be received from another system incommunication with the storage space transfer tool 250. The process 500includes identifying the first storage space 510, such as via a pointerto a physical storage device and/or sectors of a physical storage devicewhere the first storage space is located.

In some implementations, upon receiving the request to transfer astorage space from a first storage space to a second, destinationstorage space, the storage space transfer system 260 may optionallyverify that certain pre-requisites for the transfer have been satisfied.For example, in some embodiments, the process 500 can optionally includedetermining if the second storage space has been established 515, suchas via a pointer to a physical storage device and/or sectors of aphysical storage device where the second storage space is located. Ifthe second storage space has not been established, the process 500 caninclude allocating a second storage space 520. The allocation mayinclude setting a pointer to a second physical storage device on whichthe second storage space is to be stored and/or a pointer to a differentlocation on the first physical storage device.

Additionally, the storage space transfer system 260 may determine thatthe second, destination storage space is accessible and/or has storagecapacity for the data from the first storage space. The storage spacetransfer system 260 may perform additional and/or other checks to verifythe suitability of the second storage space. In other embodiments, theabove checks may be skipped. The process 500 further includesidentifying the second storage space 525, such as via the pointersdescribed above indicating the physical storage device locations for thesecond storage space.

Although the storage space transfer system 260 have been described aboveas performing pre-requisite checks, in other embodiments, anothercomponent of the storage space transfer tool 250 may perform thesechecks. In such cases, the storage space transfer system 260 can startthe transfer process upon receiving an indication from such othercomponent that the pre-requisite checks have been satisfactorilycompleted. Thus, upon confirming that all pre-requisite checks for thetransfer are satisfactorily completed or if the pre-requisite checks arenot used, the storage space transfer system 260 starts the transfer ofdata from the first storage space to the second storage space. Duringtransfer process, a user, such as a virtual machine or other computingdevice, can keep running in a normal manner by accessing and/ormodifying any data that is not being transferred by a copier 266. Thus,the operations described herein do not significantly impact the use ofthe underlying data during the transfer. In fact, the user may continueoperating normally during a significant portion of the transfer process.

The process 500 includes creating an access table 262 and a locationtable 264 to be referenced during the process 500 for copying and/orusing data during the transfer process 500. The data represented by theaccess ranges 310, location ranges 410, access values 300, and locationvalues 400 can be associated with one or more portions of data stored inthe first storage space, such as block sectors, files, bytes, etc. Thedata represented by the ranges 310, 410 and values 300, 400 can alsovary in size, though the data represented by corresponding access ranges310 and location ranges 410 and/or access values 300 and location values400 are the same size. Initially the access values 300 are all set to afirst access value, such as 0 bit value, indicative of the data notcurrently being in use. Similarly, the location values 400 are all setto a first location value, such as 0 bit value, indicative of the datacurrently being located in the first storage device. If data isaccessed, either by a user or by the copier, then the storage spacetransfer system 260 sets a corresponding access value 300 to a secondaccess value, such as a 1 bit value, indicative of the underlying databeing in use.

The process 500 includes requesting a next access range 310 in theaccess table 262, 535, and determining if any of the access values 300indicate that data within the access range 310 is in use 540. If theaccess values 300 in the access range 310 indicate that the underlyingdata is not in use, then the process 500 proceeds to determine if thecorresponding location values 400 for a corresponding location range 410indicate that the underlying data is located at the second storage space545. The location values 400 can be a first location value, such as a 0bit value, if the underlying data is located in the first storage spaceand a second location value, such as a 1 bit value, if the underlyingdata is located in the second storage space. In some implementations, aseparate map of pointers to specific physical locations of theunderlying data can be maintained to be accessed to retrieve and/ormodify the stored data.

If the location values 400 for the corresponding location range 410indicates that the underlying data has already been transferred to thesecond storage space, then the process 500 returns to request 535 thenext access range 310 in the access table 262. If all or part of theunderlying data has not been transferred from the first storage space tothe second storage space based on the location values 400 of thecorresponding location range 410 in the location table 264, then theprocess 500 proceeds to set the access values 300 for the access range310 to the second access value indicative of the underlying data beingin use for copying. The copier 266 copies the underlying data from thefirst storage space to the second storage space 555. Once the copier 266has completed copying the underlying data, each of the location values400 of the corresponding location range 410 of the location table 264are modified to indicate the underlying data is now stored in the secondstorage space 560.

The process 500 proceeds to reset the access values 300 of thecorresponding access range 310 to the first access value to indicate theunderlying data is no longer in use. During the preceding operations,550 through 565, if a request to access the underlying data is receivedduring the copying, the request can be queued in a waiting list. Thestorage space transfer system 260 can respond to the queued waiting listrequests after the access values 300 are reset to allow the user toaccess the underlying data from the new second storage space location.

The process 500 includes determining if all the location values 400 inthe location table 264 have been updated to indicate all the underlyingdata is transferred to the second storage space 570. If there arelocation values 400 indicating data is still stored in the first storagespace, the process 500 returns to request a new access range 310 in theaccess table 262 at operation 535. If all location values 400 indicateall the underlying data has been transferred to the second storagespace, the process 500 proceeds to end 575.

Since the user, such as a virtual computing system or other computingsystem, is still running and reading or writing data, the storage spacetransfer system 260 can also control the reading and writing based onthe location of the underlying data using the location table 264. FIG. 6depicts a process 600 to read and/or write data during the transferprocess 500. Additional, fewer, or different operations may be performedin the process 600 depending on the embodiment. The process 600 starts605 by receiving a data access request 610. A determination of whetherthe access request is a read request 615 is made. If the request is aread request, then the location table 264 is accessed 620 and thecorresponding location value 400 for the requested data is used 625 forreading the underlying data. If the responded value is a first locationvalue, such as a 0 bit value, then the underlying data is stored in thefirst storage space. If the responded value is a second location value,such as a 1 bit value, then the underlying data is stored in the secondstorage space.

If the request received is not a read request, such as a write request,then the location table 264 is accessed 620 and a determination 635 ismade whether the corresponding location value 400 corresponds to asecond location value indicative of the data being stored in the secondstorage space. If the location value 400 is the second location value,then the write request is used to write to the second storage space 640.If the location value 400 is the first location value, then the accessvalue 300 for the underlying data in the access table 262 is set to asecond access value, such as a 1 bit value, indicative of the underlyingdata being in use 645. The write request is then used to write to thefirst storage space 650. When the write request is finished, then theaccess value 300 for the underlying data in the access table 262 is setto a first access value, such as a 0 bit value, indicative of theunderlying data no longer being in use 655. The process 600 ends 660. Insome implementations, the process 600 can repeat for each data accessrequest.

By allowing access to the underlying data during the data transferprocess 500 using the process 600, downtime for a user, such as avirtual machine, is greatly reduced in that the computing system maycontinue operating during the transfer process. Thus, the transfer ofdata from the first storage space to a second storage space can occur asa background process that has minimal impact on the operation of thecomputing system. Moreover, by selecting small portions of data for theranges 310, 410, the delay for requests to access the underlying datacan be reduced to a minimum where the data transfer speed is high andthe incremental transferred data ranges are small.

Thus, the present disclosure provides a system and method fortransferring data from a first storage space to a second storage spacein an efficient, easy, and automatic manner. The downtime for a user,such as a virtual machine or another computing device, during the datatransfer process is minimized while permitting use of the underlyingdata that is not in the process of being transferred.

Although the present disclosure has been described with respect tosoftware applications, in other embodiments, one or more aspects of thepresent disclosure may be applicable to other components of the virtualcomputing system 100 that may be suitable for real-time monitoring bythe user.

It is also to be understood that in some embodiments, any of theoperations described herein may be implemented at least in part ascomputer-readable instructions stored on a computer-readable memory.Upon execution of the computer-readable instructions by a processor, thecomputer-readable instructions may cause a node to perform theoperations.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected,” or“operably coupled,” to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable,” to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents and/or wirelessly interactable and/or wirelessly interactingcomponents and/or logically interacting and/or logically interactablecomponents.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, and C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., “a system having at leastone of A, B, or C” would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.” Further, unlessotherwise noted, the use of the words “approximate,” “about,” “around,”“substantially,” etc., mean plus or minus ten percent.

The foregoing description of illustrative embodiments has been presentedfor purposes of illustration and of description. It is not intended tobe exhaustive or limiting with respect to the precise form disclosed,and modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. A method comprising: receiving, by a storagespace transfer system, a request to transfer first data from a firststorage space to a second storage space; receiving, by the storage spacetransfer system, a write request during the transfer of the first datato write second data to a location; determining, by the storage spacetransfer system, from an access data structure that the location is notin use; determining, by the storage space transfer system, from alocation data structure whether the location is in the first storagespace or the second storage space; and writing, by the storage spacetransfer system, the second data in the first storage space if thelocation is in the first storage space or in the second storage space ifthe location is in the second storage space.
 2. The method of claim 1,wherein the access data structure is a bit map.
 3. The method of claim1, wherein the location data structure is a bit map.
 4. The method ofclaim 1 further comprising: determining, by the storage space transfersystem and in response to receiving a read request to read third data,whether the third data is located in the first storage space or thesecond storage space; and reading, by the storage space transfer system,the third data from the first storage space if the third data is locatedin the first storage space or from the second storage space if the thirddata is located in the second storage space.
 5. The method of claim 1,further comprising: queuing, by the storage space transfer system, thewrite request until the location is not in use.
 6. The method of claim1, wherein the storage space transfer system is part of a hypervisor. 7.The method of claim 1, wherein the storage space transfer system is partof a virtual machine.
 8. The method of claim 1, wherein at least one ofthe first storage space or the second storage space is an allocatedstorage space for a virtual machine.
 9. The method of claim 1, whereinone of the first storage space or the second storage space is a sharedstorage space.
 10. A system comprising: a storage space transfer systemto: receive a request to transfer first data from a first storage spaceto a second storage space; receive a write request during the transferof the first data to write second data to a location: determine from anaccess data structure that the location is not in use; determine from alocation data structure whether the location is in the first storagespace or the second storage space; and write the second data in thefirst storage space if the location is in the first storage space or inthe second storage space if the location is in the second storage space.11. The system of claim 10, wherein the storage space transfer system ispart of a controller/service virtual machine or a hypervisor or avirtual machine.
 12. The system of claim 10, wherein at least one of thefirst storage space or the second storage space is an allocated storagespace for a virtual machine.
 13. The system of claim 10, wherein thefirst storage space is a local virtual disk and the second storage spaceis a shared storage space that is shared by multiple host machines. 14.The system of claim 10, wherein at least one of the access datastructure or the location data structure is a bit map.
 15. Anon-transitory computer readable memory storing computer program code tocause a computer to perform a method comprising: receiving a request totransfer first data from a first storage space to a second storagespace; receiving a write request during the transfer of the first datato write second data to a location; determining from an access datastructure that the location is not in use; determining from a locationdata structure whether the location is in the first storage space or thesecond storage space; and writing the second data in the first storagespace if the location is in the first storage space or in the secondstorage space if the location is in the second storage space.
 16. Thenon-transitory computer readable memory of claim 15, further comprisingprogram code to create the access data structure on the first storagespace before the transfer of the first data.
 17. The non-transitorycomputer readable memory of claim 15, further comprising program code tocreate the location data structure on the first storage space before thetransfer of the first data.
 18. The non-transitory computer readablememory of claim 15, further comprising program code to: set an accessvalue corresponding to the location to a first value in the access datastructure before the writing of the second data; and set the accessvalue corresponding to the location to a second value after the writingof the second data.
 19. The non-transitory computer readable memory ofclaim 15, further comprising program code to: queue the write requestuntil the first data is no longer in use.
 20. The non-transitorycomputer readable memory of claim 15, further comprising program codeto: determine, in response to a read request to read third data, whetherthe third data is located in the first storage space or the secondstorage space; and read the third data from the first storage space ifthe third data is located in the first storage space or from the secondstorage space if the third data is located in the second storage space.21. The system of claim 10, wherein the storage space transfer systemfurther: determines, in response to a read request to read third data,whether the third data is located in the first storage space or thesecond storage space; and reads the third data from the first storagespace if the third data is located in the first storage space or fromthe second storage space if the third data is located in the secondstorage space.
 22. The method of claim 1 further comprising: changing,by the storage space transfer system, an access value in the access datastructure indicating that the location is in use before writing thesecond data; and changing, by the storage space transfer system, theaccess value after writing the second data indicating that the locationis not in use.
 23. The method of claim 1, further comprising:determining, by the storage space transfer system, the location as beingin the first storage space or the second storage space based upon alocation value corresponding to the location in the location datastructure.